1. Field of the Invention
The present invention relates to an air-conditioning apparatus for the electric vehicle having a compressor driven by the electric power supplied from a battery.
2. Description of the Related Art
In an air-conditioning apparatus for the electric vehicle, the device for driving an electrically-operated compressor for air-conditioning purposes supplies a large current to the electrically-operated compressor. Therefore, the large electric noise interferes with the car radio, and the large current fluctuations adversely affects the durability of the battery for supplying the current. As a result, it is necessary, as shown in the current waveform diagrams of FIG. 5(a) and (b), to smooth the current supplied to the electrically-driven compressor thereby to suppress the electric noise and current fluctuations. For this purpose, a capacitor of large electrostatic capacity is provided at a point where the electrically-operated compressor driving means is supplied with electric power through a switching device from the battery.
The provision of a capacitor with a large electrostatic capacity, however, causes a large charge current to flow to the capacitor on the instant the battery is connected. The resulting problem is that the fuse blows out or the pattern wiring of the printed board is broken, or a surge current is generated by the resonance with the inductance component of the wiring from the battery as shown by the waveform diagram of FIG. 5(c), thereby damaging the electrically-operated compressor driving means.
In order to suppress the charging surge current to the capacitor on the instant the battery is connected, the capacitor is charged through a conducting device. Normally, a resistor is used as the conducting device.
The operation of a conventional air-conditioning apparatus for the electric vehicle will be explained with reference to the circuit diagram of FIG. 7 showing a conventional air-conditioning apparatus for the electric vehicle and the circuit operation diagram for the air-conditioning apparatus for the electric vehicle shown in FIG. 8. With the battery 2 connected, the voltage Vc of the capacitor 11, the current Ir of the resistor 8 and the current Ic of the capacitor 11 are determined as shown in FIG. 8 from the time constant based on the resistance value of the resistor 8 and the electrostatic capacity of the capacitor 11. When the capacitor 11 is charged and input voltage detector means 7 detects a voltage of more than a predetermined value, a controller 6 closes the switching device 3, and power is supplied through the switching device 3 from the battery, thereby causing the electrically-operated compressor driving device to drive the electrically-operated air-conditioning compressor.
The resistance value of the resistor 8 is required to be sufficiently large to suppress the charging surge current as described above. On the other hand, the resistance value of the resistor 8 must be small enough to blow out the fuse at the time of a malfunction such as the damage to the electrically-operated compressor driving device or the shortening of the capacitor. The value of the fuse is about 10 A to blow out positively at the time of a malfunction. The resistance value of the resistor 8 becomes 7.5 ohm if the voltage across the battery 2 is 300 V to assure current flow of 40 A to blow out the fuse positively at the time of a malfunction. When the battery 2 is connected, therefore, the current of about 40 A flows, the subsequent current being determined by the time constant. The larger the electrostatic capacity of the capacitor 11, the larger the time constant, thereby increasing the power consumption of the resistor 8 accordingly. The electrostatic capacity of the capacitor 11 normally assumes a large value of about 1000 .mu.F for smoothing the current. At the time of connecting the battery 2, the instantaneous maximum power consumption of the resistor 8 is 300 V.times.40 A=12,000 W. A large-power resistor 8 must therefore be used to maintain the reliability thereof. According to the guaranteed specification of the resistor 8, the resistor 8 is a rated product of about 60 W.
As seen from FIGS. 6A and 6B showing the form of the resistor, a large space is required for the controller of the vehicle air-conditioning apparatus even when two 30 W resistors instead of a 60 W resistor are used.
In the case where the electrically-operated compressor driving device or capacitor is in a minor trouble such as when the current of 9 A flows with the fuse not blown out, the resistor 8 consumes large electric power of (9 A)2.times.7.5 ohm=607.5 W, thereby compromising the reliability due to the smoking, solder melting or wiring damage due to overheating.
In the case where a resistor is used as a conducting device as described above, such a resistor must be a large-power item of about 60 W. The large-power resistor, due to the large size, can not be directly connected on the printed circuit board in the electrically-operated compressor driving device, and requires a considerable space to be secured for mounting, thereby leading to a bulky air-conditioning apparatus. The air-conditioning apparatus for the electric vehicle must be installed in a limited space in the vehicle, and therefore must be as small as possible. A conducting device is required which does not cause an increased size of the air-conditioning apparatus.
In the case where a resistor is used as a conducting device, a wiring is required of the resistor, and further the wiring must be fixed in order not to be damaged by contact with a heat generating material or a metal edge. The resulting complicated and bulky structure of a conducting device makes fabrication difficult.
Also, the resistance value of the resistor is required to be sufficiently large to suppress the charging surge current. On the other hand, the resistance value must be small enough to allow the fuse to blow out when the electrically-operated compressor driving device is damaged or the capacitor is shorted. The resistance value of the resistor, therefore, must be changed in accordance with the battery voltage or the electrostatic capacity of the capacitor. Since the battery voltage of the electric vehicle is so varied and the capacity required of the air-conditioning apparatus varies from one vehicle to another, different amounts of current are supplied to different electrically-operated compressors. It is therefore necessary to change the resistance value of the resistor for different air-conditioning apparatuses or vehicles, with the result that an increased number of types of air-conditioning apparatus complicates the production control, often raising the problem of an increased cost or other production-related problems.
Also, when the electrically-operated compressor driving device or the capacitor is in minor trouble with the fuse not blown out, the resistor consumes large electric power for a reduced reliability due to overheat. As a result, the measure taken for heat radiation undesirably increases the size of the air-conditioning apparatus.
Further, the capacitor, when connected in reverse polarities, is "punctured" to damage the apparatus. When a switching power supply is connected in parallel to the capacitor, a sharp voltage increase may damage the switching power circuit.